1. Field of the Invention
The present invention relates to a mobile phone apparatus.
2. Description of the Prior Art
A known type of mobile phone transmits and receives on separate frequencies in the UHF (Ultra High Frequency) band. Such phones are used in the TDMA [Time Division Multiple Access] (GSM [Global System for Mobile communication]) systems, for which there are stringent specifications on the amount of transmitter noise which falls within the receiver band. A known technique for meeting this specification uses a high Q (quality factor) notch filter at the output of the power amplifier of the phone transmitter. However, such a filter is relatively large and has a significant insertion loss. In order to compensate for this loss, the power amplifier has to provide a higher output power and therefore draws a higher current from the power supply, which is normally a battery within the phone. Thus, for a given size of battery, the talk time of the phone is reduced.
In another known type of phone, a phase locked loop is connected between a modulator which produces a signal to be transmitted in the transmit band and the power amplifier of the phone. The modulator output is connected to an input of the phase sensitive detector of the phase locked loop and the input of the power amplifier is connected to the voltage controlled oscillator of the loop. The wide band noise of the transmitter signal supplied to the power amplifier is then determined by the phase noise of the voltage controlled oscillator when the loop is locked. However, low phase noise voltage controlled oscillators for operation at UHF are relatively expensive.